How to check relative humidity on a hot day to keep yourself cool

Source: The post is based on the article “How to check relative humidity on a hot day to keep yourself cool” published in The Hindu on 25th April 2023

What is the News?

Relative humidity is a simple concept as far as weather phenomena go but it has significant, far-reaching consequences for how we must take care of ourselves on a hot or wet day.

What is Humidity?

Humidity is a measurement of the amount of water vapour in the air. There are three ways to track humidity:

Absolute humidity: It is the mass of water vapour in a given volume of air and water vapour mixture expressed as kg/m3.

Specific humidity: It is equal to the mass of the moisture divided by the mass of air. It is expressed as a dimensionless number (but sometimes as grams per kilogram among other similar units).

Relative humidity: It is important as it factors in the amount of vapour that air can hold at different temperatures. It is determined by dividing the vapour density of the air by saturation vapour density at dry-bulb temperature.

Why does relative humidity matter?

The higher the relative humidity of air, the more it is filled with moisture. When air already contains a lot of moisture, the sweat on your skin can’t evaporate. 

At the same time, the body keeps sweating as it is still expecting to cool itself. As a result, if the relative humidity is high, you can sweat on a hot day even when you are sitting still while your body keeps accumulating heat. This can quickly become dangerous.

Relative humidity of 30-60% is generally considered to be comfortable. Environments that have lower levels than this typically use humidifiers to increase the humidity. When the level is higher, a fan will help move the air around you and help sweat evaporate better.

What is Wet Bulb Temperature?

Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. 

It is a limit that considers heat and humidity beyond which humans can not tolerate high temperatures.

It is measured by wrapping a wet cloth on a thermometer and observing the temperature at which evaporation occurs. This is distinct from the more commonly known ambient or dry temperature, which is measured as is.

The highest acceptable wet-bulb temperature for humans is 35 °C (95 °F) for six hours. Above that point, even healthy people begin to develop serious, often deadly health problems.